Erythropoietin administration in humans causes a marked and prolonged reduction in circulating hepcidin

Optimal iron fortification of maternal diet during pregnancy and nursing for investigating and preventing iron deficiency in young rhesus monkeys

The realization that pregnant and infant monkeys were challenged by high nutritional needs for iron led vendors to markedly increase iron concentrations in commercial diets. Yet, no systematic research was conducted to investigate the consequences of this important dietary change. Hematology and iron panels were determined for 142 infant rhesus monkeys gestated and reared on 3 different diets varying in iron concentration (180, 225 or 380 mg Fe/kg). Anemia was significantly more prevalent in offspring from females fed the 180 and 225 mg Fe/kg diets (32-41% versus 0 for the 380 mg Fe/kg diet, P<0.001). Higher hepcidin levels were protective against iron overload in infants from the 380 mg Fe/kg condition. These findings indicate a highly fortified diet during pregnancy continues to have postnatal benefits for the growing infant. However, for those interested in iron deficiency, lower iron diets provide a reliable way to generate anemic infant monkeys for research.

Regulation of erythropoiesis by hypoxia-inducible factors

A classic physiologic response to systemic hypoxia is the increase in red blood cell production. Hypoxia-inducible factors (HIFs) orchestrate this response by inducing cell-type specific gene expression changes that result in increased erythropoietin (EPO) production in kidney and liver, in enhanced iron uptake and utilization and in adjustments of the bone marrow microenvironment that facilitate erythroid progenitor maturation and proliferation. In particular HIF-2 has emerged as the transcription factor that regulates EPO synthesis in the kidney and liver and plays a critical role in the regulation of intestinal iron uptake. Its key function in the hypoxic regulation of erythropoiesis is underscored by genetic studies in human populations that live at high-altitude and by mutational analysis of patients with familial erythrocytosis. This review provides a perspective on recent insights into HIF-controlled erythropoiesis and iron metabolism, and examines cell types that have EPO-producing capability. Furthermore, the review summarizes clinical syndromes associated with mutations in the O(2)-sensing pathway and the genetic changes that occur in high altitude natives. The therapeutic potential of pharmacologic HIF activation for the treatment of anemia is discussed.

Administration of recombinant erythropoietin alone does not improve the phenotype in iron refractory iron deficiency anemia patients

Mutations in transmembrane protease, serine 6 (TMPRSS6) cause iron refractory iron deficiency anemia (IRIDA). Parenteral iron administration may slightly improve hemoglobin level but is troublesome for patients. Optimal treatment has yet to be determined. We identified five patients from four independent families displaying the IRIDA picture with truncating biallelic mutations in TMPRSS6, one of which is novel. Liver iron determined by superconducting quantum interference device biosusceptometry ranged from 390 to 720 µg Fe/g wet weight (normal range 100-500; n = 3). Intestinal iron absorption (12 and 32 %, normal range 10-50; n = 2) and 59Fe erythrocyte incorporation after ingestion of 59Fe (57 and 38 %, normal range 70-90; n = 2) were inadequately low for iron-deficient anemic individuals. Baseline serum erythropoietin was elevated or borderline high in four patients. Administration of recombinant human erythropoietin (rhEPO) at up to 273 and 188 U/kg body weight/week alone did not improve anemia or result in a decrease of urinary hepcidin in two individuals. In conclusion, the ability of exogenous rhEPO to increase hemoglobin level appears to be impaired in IRIDA.

Erythropoietin stimulation decreases hepcidin expression through hematopoietic activity on bone marrow cells in mice

Erythropoiesis-stimulating agents (ESA) are now central to the treatment of renal anemia and are associated with improved clinical outcomes. It is well known that erythropoietin (EPO) is a key regulator of erythropoiesis through its promotion of red blood cell production. In order to investigate the role of ESA on iron metabolism, we analyzed the regulation of the iron regulatory hormone hepcidin by ESA treatment in a bone marrow transplant model in mouse. After treating C57BL/6 mice with continuous erythropoietin receptor activator (C.E.R.A.), recombinant human epoetin-β (rhEPO), or recombinant human carbamylated epoetin-β (rhCEPO), we investigated serum hepcidin concentrations and parameters of erythropoiesis. Serum hepcidin concentrations after rhEPO treatment were analyzed in mice subjected to total body irradiation followed by bone marrow transplantation. C.E.R.A. administration caused long-term downregulation of serum hepcidin levels. Serum hepcidin levels in rhEPO-treated mice decreased significantly, whereas there was no change in rhCEPO-treated mice. The reduction in circulating hepcidin levels after rhEPO administration was not observed in irradiated mice. Finally, bone marrow transplantation recovered the response to rhEPO administration that downregulates hepcidin concentration in irradiated mice. These results indicate that ESA treatment downregulates serum hepcidin concentrations, mainly by indirect mechanisms affecting hematopoietic activity in bone marrow cells.

Hypoxia-inducible factor regulates hepcidin via erythropoietin-induced erythropoiesis

Iron demand in bone marrow increases when erythropoiesis is stimulated by hypoxia via increased erythropoietin (EPO) synthesis in kidney and liver. Hepcidin, a small polypeptide produced by hepatocytes, plays a central role in regulating iron uptake by promoting internalization and degradation of ferroportin, the only known cellular iron exporter. Hypoxia suppresses hepcidin, thereby enhancing intestinal iron uptake and release from internal stores. While HIF, a central mediator of cellular adaptation to hypoxia, directly regulates renal and hepatic EPO synthesis under hypoxia, the molecular basis of hypoxia/HIF-mediated hepcidin suppression in the liver remains unclear. Here, we used a genetic approach to disengage HIF activation from EPO synthesis and found that HIF-mediated suppression of the hepcidin gene (Hamp1) required EPO induction. EPO induction was associated with increased erythropoietic activity and elevated serum levels of growth differentiation factor 15. When erythropoiesis was inhibited pharmacologically, Hamp1 was no longer suppressed despite profound elevations in serum EPO, indicating that EPO by itself is not directly involved in Hamp1 regulation. Taken together, we provide in vivo evidence that Hamp1 suppression by the HIF pathway occurs indirectly through stimulation of EPO-induced erythropoiesis.

Management of anemia of inflammation in the elderly

Anemia of any degree is recognized as a significant independent contributor to morbidity, mortality, and frailty in elderly patients. Among the broad types of anemia in the elderly a peculiar role seems to be played by the anemia associated with chronic inflammation, which remains the most complex form of anemia to treat. The origin of this nonspecific inflammation in the elderly has not yet been clarified. It seems more plausible that the oxidative stress that accompanies ageing is the real cause of chronic inflammation of the elderly and that the same oxidative stress is actually a major cause of this anemia. The erythropoietic agents have the potential to play a therapeutic role in this patient population. Despite some promising results, rHuEPO does not have a specific indication for the treatment of anemia in the elderly. Moreover, concerns about their side effects have spurred the search for alternatives. Considering the etiopathogenetic mechanisms of anemia of inflammation in the elderly population, an integrated nutritional/dietetic approach with nutraceuticals that can manipulate oxidative stress and related inflammation may prevent the onset of this anemia and its negative impact on patients' performance and quality of life.

Circulating human hepcidin-25 concentrations display a diurnal rhythm, increase with prolonged fasting, and are reduced by growth hormone administration

BACKGROUND

Hepcidin-25 reduces iron absorption by binding to the intestinal iron transporter ferroportin and causing its degradation. Currently, little is known about the basal regulation of circulating hepcidin-25. In addition, although erythropoietin administration has been reported to decrease the circulating hepcidin concentration, information is limited regarding how other stimulators of erythropoiesis, such as growth hormone (GH), might alter hepcidin-25 concentrations.

METHODS

We used a sensitive and specific hepcidin-25 dual-monoclonal antibody sandwich immunoassay to measure hepcidin-25 in healthy human volunteers at various time points throughout the day and during 3 days of fasting and subsequent refeeding. We also measured hepcidin-25 concentrations in healthy volunteers after GH administration.

RESULTS

In healthy individuals, hepcidin-25 concentrations displayed a diurnal variation, with concentrations being lowest in the early morning and steadily increasing throughout the day before declining during the evening hours, a pattern that was not influenced by food intake. Prolonged fasting produced statistically significant increases in hepcidin-25 concentrations. Refeeding reversed this process, and GH administration markedly decreased hepcidin-25 concentrations.

CONCLUSIONS

Our results indicate that in humans, hepcidin-25 exhibits diurnal changes that can be altered by prolonged fasting, which increases hepcidin-25 concentrations approximately 3-fold after 3 days of fasting, possibly owing to a suppression of erythropoiesis that may occur during the fasting state to preserve tissue iron concentrations. In contrast, GH administration decreased hepcidin-25 concentrations by approximately 65%, presumably by stimulating erythropoiesis. These results indicate that circulating hepcidin-25 concentrations display much more dynamic and rapid variation than might have been anticipated previously.

Treatment of erythropoietin deficiency in mice with systemically administered siRNA

Anemia linked to a relative deficiency of renal erythropoietin production is a significant cause of morbidity and medical expenditures in the developed world. Recombinant erythropoietin is expensive and has been linked to excess cardiovascular events. Moreover, some patients become refractory to erythropoietin because of increased production of factors such as hepcidin. During fetal life, the liver, rather than the kidney, is the major source of erythropoietin. In the present study, we show that it is feasible to reactivate hepatic erythropoietin production and suppress hepcidin levels using systemically delivered siRNAs targeting the EglN prolyl hydroxylases specifically in the liver, leading to improved RBC production in models of anemia caused by either renal insufficiency or chronic inflammation with enhanced hepcidin production.

Erythropoietin-driven signaling ameliorates the survival defect of DMT1-mutant erythroid progenitors and erythroblasts

BACKGROUND

Hypochromic microcytic anemia associated with ineffective erythropoiesis caused by recessive mutations in divalent metal transporter 1 (DMT1) can be improved with high-dose erythropoietin supplementation. The aim of this study was to characterize and compare erythropoiesis in samples from a DMT1-mutant patient before and after treatment with erythropoietin, as well as in a mouse model with a DMT1 mutation, the mk/mk mice.

DESIGN AND METHODS

Colony assays were used to compare the in vitro growth of pre-treatment and post-treatment erythroid progenitors in a DMT1-mutant patient. To enable a comparison with human data, high doses of erythropoietin were administered to mk/mk mice. The apoptotic status of erythroblasts, the expression of anti-apoptotic proteins, and the key components of the bone marrow-hepcidin axis were evaluated.

RESULTS

Erythropoietin therapy in vivo or the addition of a broad-spectrum caspase inhibitor in vitro significantly improved the growth of human DMT1-mutant erythroid progenitors. A decreased number of apoptotic erythroblasts was detected in the patient's bone marrow after erythropoietin treatment. In mk/mk mice, erythropoietin administration increased activation of signal transducer and activator of transcription 5 (STAT5) and reduced apoptosis in bone marrow and spleen erythroblasts. mk/mk mice propagated on the 129S6/SvEvTac background resembled DMT1-mutant patients in having increased plasma iron but differed by having functional iron deficiency after erythropoietin administration. Co-regulation of hepcidin and growth differentiation factor 15 (GDF15) levels was observed in mk/mk mice but not in the patient.

CONCLUSIONS

Erythropoietin inhibits apoptosis of DMT1-mutant erythroid progenitors and differentiating erythroblasts. Ineffective erythropoiesis associated with defective erythroid iron utilization due to DMT1 mutations has specific biological and clinical features.

Chronic anemia and effects of iron supplementation in a research colony of adult Rhesus macaques (Macaca mulatta)

A cohort of rhesus macaques used in neuroscience research was found at routine examinations to have chronic anemia (spun Hct less than 30%). Four anemic (Hct, 24.8% ± 3.4%) and 10 control (39.6% ± 2.9%) macaques were assessed to characterize the anemia and determine probable cause(s); some animals in both groups had cephalic implants. Diagnostic tests included CBC, bone marrow evaluations, iron panels, and serum erythropoietin and hepcidin concentrations. Serum iron and ferritin were 15.8 ± 11.1 μg/dL and 103.8 ± 53.1 ng/mL, respectively, for the anemic group compared with 109.8 ± 23.8 μL/dL and 88.5 ± 41.9 ng/mL, respectively, for the control group. Erythropoietin levels were 16.2 to over 100 mU/mL for the anemic macaques compared with 0 to 1.3 mU/mL for the control group. Hepcidin results were similar in both groups. Because the findings of low iron, high erythropoietin, and normal hepcidin in the anemic macaques supported iron-deficiency anemia or anemia of chronic disease combined with iron-deficiency anemia, a regimen of 4 doses of iron dextran was provided. In treated macaques, Hct rose to 36.3% ± 6.8%, serum iron levels increased to 94.0 ± 41.9 μg/dL, and erythropoietin levels fell to 0.15 to 0.55 mU/mL. Maintenance of normal Hct was variable between macaques and reflected individual ongoing clinical events.

The evolving science of detection of 'blood doping'

Blood doping practices in sports have been around for at least half a century and will likely remain for several years to come. The main reason for the various forms of blood doping to be common is that they are easy to perform, and the effects on exercise performance are gigantic. Yet another reason for blood doping to be a popular illicit practice is that detection is difficult. For autologous blood transfusions, for example, no direct test exists, and the direct testing of misuse with recombinant human erythropoietin (rhEpo) has proven very difficult despite a test exists. Future blood doping practice will likely include the stabilization of the transcription factor hypoxia-inducible factor which leads to an increased endogenous erythropoietin synthesis. It seems unrealistic to develop specific test against such drugs (and the copies hereof originating from illegal laboratories). In an attempt to detect and limit blood doping, the World Anti-Doping Agency (WADA) has launched the Athlete Biological Passport where indirect markers for all types of blood doping are evaluated on an individual level. The approach seemed promising, but a recent publication demonstrates the system to be incapable of detecting even a single subject as 'suspicious' while treated with rhEpo for 10-12 weeks. Sad to say, the hope that the 2012 London Olympics should be cleaner in regard to blood doping seems faint. We propose that WADA strengthens the quality and capacities of the National Anti-Doping Agencies and that they work more efficiently with the international sports federations in an attempt to limit blood doping.

Hepatic hypoxia-inducible factor-2 down-regulates hepcidin expression in mice through an erythropoietin-mediated increase in erythropoiesis

BACKGROUND

Iron metabolism, regulated by the iron hormone hepcidin, and oxygen homeostasis, dependent on hypoxia-inducible factors, are strongly interconnected. We previously reported that in mice in which both liver hypoxia-inducible factors-1 and -2 are stabilized (the hepatocyte von Hippel-Lindau knockout mouse model), hepcidin expression was strongly repressed and we hypothesized that hypoxia-inducible factor-2 could be the major regulatory component contributing to the hepcidin down-regulation.

DESIGN AND METHODS

We generated and analyzed hepatocyte-specific knockout mice harboring either hypoxia-inducible factor-2α deficiency (Hif2a knockout) or constitutive hypoxia-inducible factor-2α stabilization (Vhlh/Hif1a knockout) and ex vivo systems (primary hepatocyte cultures). Hif2a knockout mice were fed an iron-deficient diet for 2 months and Vhlh/Hif1a knockout mice were treated with neutralizing erythropoietin antibody.

RESULTS

We demonstrated that hypoxia-inducible factor-2 is dispensable in hepcidin gene regulation in the context of an adaptive response to iron-deficiency anemia. However, its overexpression in the double Vhlh/Hif1a hepatocyte-specific knockout mice indirectly down-regulates hepcidin expression through increased erythropoiesis and erythropoietin production. Experiments in primary hepatocytes confirmed the non-autonomous role of hypoxia-inducible factor-2 in hepcidin regulation.

CONCLUSIONS

While our results indicate that hypoxia-inducible factor-2 is not directly involved in hepcidin repression, they highlight the contribution of hepatic hypoxia-inducible factor-2 to the repression of hepcidin through erythropoietin-mediated increased erythropoiesis, a result of potential clinical interest.

Timing and determinants of erythropoietin deficiency in chronic kidney disease

BACKGROUND AND OBJECTIVES

Anemia in patients with CKD is highly related to impaired erythropoietin (EPO) response, the timing and determinants of which remain unknown.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

This study measured EPO levels and studied their relation to GFR measured by 51Cr-EDTA renal clearance (mGFR) in 336 all-stage CKD patients not receiving any erythropoiesis-stimulating agent.

RESULTS

In patients with anemia defined by World Health Organization criteria (hemoglobin [Hb] <13 g/dl in men and 12 g/dl in women), EPO response to Hb level varied by mGFR level. EPO and Hb levels were negatively correlated (r=-0.22, P=0.04) when mGFR was >30 ml/min per 1.73 m(2), whereas they were not correlated when mGFR was <30 (r=0.09, P=0.3; P for interaction=0.01). In patients with anemia, the ratio of observed EPO to the level predicted by the equation for their Hb level decreased from 0.72 (interquartile range, 0.57-0.95) for mGFR ≥60 ml/min per 1.73 m(2) to 0.36 (interquartile range, 0.16-0.69) for mGFR <15. Obesity, diabetes with nephropathy other than diabetic glomerulopathy, absolute iron deficiency, and high C-reactive protein concentrations were associated with increased EPO levels, independent of Hb and mGFR.

CONCLUSIONS

Anemia in CKD is marked by an early relative EPO deficiency, but several factors besides Hb may persistently stimulate EPO synthesis. Although EPO deficiency is likely the main determinant of anemia in patients with advanced CKD, the presence of anemia in those with mGFR >30 ml/min per 1.73 m(2) calls for other explanatory factors.

Hepcidin in human iron disorders: diagnostic implications

BACKGROUND

The peptide hormone hepcidin plays a central role in regulating dietary iron absorption and body iron distribution. Many human diseases are associated with alterations in hepcidin concentrations. The measurement of hepcidin in biological fluids is therefore a promising tool in the diagnosis and management of medical conditions in which iron metabolism is affected.

CONTENT

We describe hepcidin structure, kinetics, function, and regulation. We moreover explore the therapeutic potential for modulating hepcidin expression and the diagnostic potential for hepcidin measurements in clinical practice.

SUMMARY

Cell-culture, animal, and human studies have shown that hepcidin is predominantly synthesized by hepatocytes, where its expression is regulated by body iron status, erythropoietic activity, oxygen tension, and inflammatory cytokines. Hepcidin lowers serum iron concentrations by counteracting the function of ferroportin, a major cellular iron exporter present in the membrane of macrophages, hepatocytes, and the basolateral site of enterocytes. Hepcidin is detected in biologic fluids as a 25 amino acid isoform, hepcidin-25, and 2 smaller forms, i.e., hepcidin-22 and -20; however, only hepcidin-25 has been shown to participate in the regulation of iron metabolism. Reliable assays to measure hepcidin in blood and urine by use of immunochemical and mass spectrometry methods have been developed. Results of proof-of-principle studies have highlighted hepcidin as a promising diagnostic tool and therapeutic target for iron disorders. However, before hepcidin measurements can be used in routine clinical practice, efforts will be required to assess the relevance of hepcidin isoform measurements, to harmonize the different assays, to define clinical decision limits, and to increase assay availability for clinical laboratories.

Iron deficiency and raised hepcidin in idiopathic pulmonary arterial hypertension: clinical prevalence, outcomes, and mechanistic insights

OBJECTIVES

This study sought to understand the prevalence and clinical relevance of iron deficiency in patients with idiopathic pulmonary arterial hypertension (IPAH).

BACKGROUND

Iron availability influences the pulmonary vascular response to hypoxia in humans and may be significant in the pathogenesis of IPAH.

METHODS

Iron deficiency, defined by raised levels of soluble transferrin receptor (sTfR), was investigated in 98 patients with IPAH. Hepcidin and erythropoietin (EPO) levels were also measured. The effect of bone morphogenetic protein (BMP) receptor knockdown on BMP-6-stimulated hepcidin production was assessed in human hepatoma HepG2 cells. Relationships between sTfR and exercise capacity, functional class, and all-cause mortality were analyzed.

RESULTS

Circulating sTfR levels were raised in 63% of IPAH patients, indicating significant iron deficiency. Consistent with this, iron, ferritin, and transferrin saturation levels were reduced and red cell distribution width increased, without overt anemia. Hepcidin correlated inversely with sTfR and positively with increasing ferritin. Hepcidin was inappropriately raised in IPAH independent of the inflammatory marker interleukin-6. EPO levels were also raised and correlated inversely with hepcidin. BMP receptor-type 2 (BMPR2) knockdown in HepG2 cells increased BMP-6-stimulated hepcidin expression. sTfR increased with World Health Organization functional class (p < 0.05), correlated negatively with exercise capacity (p = 0.027), and values >28.1 nmol/l independently predicted survival (p = 0.011).

CONCLUSIONS

Iron deficiency is common in IPAH patients and associated with disease severity and poor clinical outcome. Inappropriately raised hepcidin levels, which impair iron absorption from the gut, may be a factor.

Hepcidin: A Critical Regulator of Iron Metabolism during Hypoxia

Iron status affects cognitive and physical performance in humans. Recent evidence indicates that iron balance is a tightly regulated process affected by a series of factors other than diet, to include hypoxia. Hypoxia has profound effects on iron absorption and results in increased iron acquisition and erythropoiesis when humans move from sea level to altitude. The effects of hypoxia on iron balance have been attributed to hepcidin, a central regulator of iron homeostasis. This paper will focus on the molecular mechanisms by which hypoxia affects hepcidin expression, to include a review of the hypoxia inducible factor (HIF)/hypoxia response element (HRE) system, as well as recent evidence indicating that localized adipose hypoxia due to obesity may affect hepcidin signaling and organismal iron metabolism.

Chuvash polycythemia VHLR200W mutation is associated with down-regulation of hepcidin expression

Hypoxia is known to reduce the expression of hepcidin, the master regulator of iron metabolism. However, it is not clear whether this response is primarily related to increased erythropoiesis driven by hypoxically stimulated erythropoietin or to a more direct effect of hypoxia on hepcidin expression. The germline loss-of-function VHL(R200W) mutation is common in Chuvashia, Russia, and also occurs elsewhere. VHL(R200W) homozygotes have elevated hypoxia-inducible factor 1α (HIF-1α) and HIF-2α levels, increased red cell mass, propensity to thrombosis, and early mortality. Ninety VHL(R200W) homozygotes and 52 controls with normal VHL alleles from Chuvashia, Russia, were studied under basal circumstances. In univariate analyses, serum hepcidin concentration was correlated positively with serum ferritin concentration and negatively with homozygosity for VHL(R200W). After adjustment for serum erythropoietin and ferritin concentrations by multiple linear regression, the geometric mean (95% confidence interval of mean) hepcidin concentration was 8.1 (6.3-10.5) ng/mL in VHL(R200W) homozygotes versus 26.9 (18.6-38.0) ng/mL in controls (P < .001). In contrast, a significant independent relationship of serum erythropoietin, hemoglobin, or RBC count with hepcidin was not observed. In conclusion, up-regulation of the hypoxic response leads to decreased expression of hepcidin that may be independent of increased erythropoietin levels and increased RBC counts.

Renal anemia of inflammation: the name is self-explanatory

BACKGROUND

Anemia is inevitable as chronic kidney disease (CKD) advances. With the advent of erythropoietin-stimulating agents (ESAs), considerable improvement has been achieved in the management of anemia. However, some patients show a reduced response to ESAs.

METHODS

Many factors affect the response to ESA treatment. CKD is now considered as an inflammatory disorder and this understanding led to the recognition of the central role of inflammation in ESA resistance. Inflammation is related to untoward outcomes, including atherosclerosis and anemia, in the CKD population. Furthermore, recognition of deleterious effects of proinflammatory markers at different levels of erythropoiesis led to a change in the name of 'anemia of chronic disease' to anemia of inflammation.

RESULTS

The discovery of hepcidin as the major controller of iron metabolism in anemia of inflammation answered many questions regarding the interaction of erythropoietin, iron and bone marrow. Hepcidin production in the liver is driven by three major factors: inflammation, iron overload and anemia/hypoxia. Hepcidin levels are increased in patients with CKD due to the interaction of many factors; a comprehensive understanding of these pathways is thus critical in the effort to alleviate anemia of inflammation and ESA resistance.

CONCLUSION

In this review, we discussed the epidemiology, determinants and consequences of anemia of inflammation in CKD patients with special emphasis on the central role of hepcidin along with molecular pathways driving its production.

Early effects of erythropoietin on serum hepcidin and serum iron bioavailability in healthy volunteers

Hepcidin regulates plasma iron bioavailability and subsequently iron availability for erythropoiesis. rHuEPO has been reported to decrease hepcidin expression in case of repeated subcutaneous injections. Thus, hepcidin level measurement could be a candidate marker for detection of rHuEPO abuse. However, when used for doping, rHuEPO can be injected intravenously and the scheme of injection is unknown. Our aim was to evaluate the early effects of a single intravenous rHuEPO injection on serum hepcidin levels. Fourteen male healthy volunteers received one intravenous injection of 50 U/Kg of rHuEPO during a placebo-controlled, randomized, double-blind, cross-over study. Serum hepcidin, quantified by a competitive ELISA method and iron parameters was then evaluated for 24 h. Serum levels of hepcidin were significantly increased 4 h after rHuEPO injection when compared with placebo injection (78.3 ± 55.5 vs. 57.5 ± 34.6 ng/ml, respectively; +36%, p < 0.05), whereas iron and transferrin saturation dramatically decreased 12 h after rHuEPO injection when compared with placebo injection (9.2 ± 3.5 vs. 15.8 ± 4.2 μg/l, respectively; -42%, p < 0.05 and 14.8 ± 5.0 vs. 26.3 ± 6.4%, respectively; -44%, p < 0.05). In addition, 12 and 24 h after rHuEPO injection serum hepcidin levels were lower compared with placebo injection (41.6 ± 27.4 vs. 56.6 ± 28.1 ng/ml after 12 h; -27%, p < 0.05 and 26.0 ± 29.6 vs. 81.2 ± 29.4 ng/ml after 24 h; -68%, p < 0.05). Intravenous injection of recombinant EPO induces a precocious and transient increase of serum hepcidin leading to a transient decrease of iron bioavailability. The transitory increase and dynamics of its concentration make difficult the practical use of hepcidin to detect rHuEPO doping.

Advancements in anemias related to chronic conditions

Anemia of chronic disease (ACD), the most frequent anemia among hospitalized patients, occurs in chronic inflammatory disorders, such as chronic infections, cancer and autoimmune diseases. Different causes contribute to ACD including diversion of iron traffic, diminished erythropoiesis, blunted response to erythropoietin, erythrophagocytosis, hematologic malignancies and solid tumors. A particular case of ACD is represented by anemia of chronic kidney disease (CKD). ACD is characterized by hyposideremia and altered iron transport. Cytokines are implicated in the ACD by reducing erythropoiesis and increasing iron sequestration in the reticuloendothelial system. The regulation of iron absorption across the epithelium of the proximal small intestine is essential for maintaining body iron concentrations within a physiologically defined range. Hepcidin controls cellular iron efflux by binding to the iron export protein ferroportin, causing ferroportin to be phosphorylated and degraded in lysosomes. Finally, hepcidin inhibits iron release from the reticulo-endothelial system. Increased expression of hepcidin leads to decreased iron absorption and iron deficient anemia. Hepcidin, therefore, is a negative regulator of iron transport in plasma. Causes of anemia in patients with CKD are multifactorial, but the most well-known cause is inadequate erythropoietin production. In these patients, anemia increases the risk of either cardiovascular disease or renal failure.

The hepcidin-ferroportin system as a therapeutic target in anemias and iron overload disorders

The review summarizes the current understanding of the role of hepcidin and ferroportin in normal iron homeostasis and its disorders. The various approaches to therapeutic targeting of hepcidin and ferroportin in iron-overload disorders (mainly hereditary hemochromatosis and β-thalassemia) and iron-restrictive anemias (anemias associated with infections, inflammatory disorders, and certain malignancies, anemia of chronic kidney diseases, and iron-refractory iron-deficiency anemia) are also discussed.

Hepcidin in human iron disorders: therapeutic implications

The discovery of hepcidin has triggered a virtual explosion of studies on iron metabolism and related disorders, the results of which have profoundly changed our view of human diseases associated with excess of iron, iron deficiency or iron misdistribution. Not only has new light been shed on the pathogenesis of these disorders, but therapeutic applications from these advances are now foreseen. The notion that hepcidin excess or deficiency may contribute to the dysregulation of iron homeostasis in hereditary and acquired iron disorders raises the possibility that hepcidin-lowering or enhancing agents may be an effective strategy for curing the main consequences of hepcidinopathies, anemia or iron overload, respectively. Experimental pre-clinical and clinical studies have shown that hepcidin antibodies, agonists or antagonists, cytokine receptor antibodies and small-molecules that modify hepcidin expression also reverse iron abnormalities in vivo, in a number of disease models. While future studies addressing safety and long-term efficacy of hepcidin-targeted treatments will clarify risks and benefits, a new era has begun based on the treatment of disorders of iron homeostasis through the modulation of its regulatory hormone, hepcidin.

Disorders of iron metabolism. Part 1: molecular basis of iron homoeostasis

IRON FUNCTIONS: Iron is an essential micronutrient, as it is required for satisfactory erythropoietic function, oxidative metabolism and cellular immune response. IRON PHYSIOLOGY: Absorption of dietary iron (1-2 mg/day) is tightly regulated and just balanced against iron loss because there are no active iron excretory mechanisms. Dietary iron is found in haem (10%) and non-haem (ionic, 90%) forms, and their absorption occurs at the apical surface of duodenal enterocytes via different mechanisms. Iron is exported by ferroportin 1 (the only putative iron exporter) across the basolateral membrane of the enterocyte into the circulation (absorbed iron), where it binds to transferrin and is transported to sites of use and storage. Transferrin-bound iron enters target cells-mainly erythroid cells, but also immune and hepatic cells-via receptor-mediated endocytosis. Senescent erythrocytes are phagocytosed by reticuloendothelial system macrophages, haem is metabolised by haem oxygenase, and the released iron is stored as ferritin. Iron will be later exported from macrophages to transferrin. This internal turnover of iron is essential to meet the requirements of erythropoiesis (20-30 mg/day). As transferrin becomes saturated in iron-overload states, excess iron is transported to the liver, the other main storage organ for iron, carrying the risk of free radical formation and tissue damage. REGULATION OF IRON HOMOEOSTASIS: Hepcidin, synthesised by hepatocytes in response to iron concentrations, inflammation, hypoxia and erythropoiesis, is the main iron-regulatory hormone. It binds ferroportin on enterocytes, macrophages and hepatocytes triggering its internalisation and lysosomal degradation. Inappropriate hepcidin secretion may lead to either iron deficiency or iron overload.

Modulation of hepcidin production during hypoxia-induced erythropoiesis in humans in vivo: data from the HIGHCARE project

Iron is tightly connected to oxygen homeostasis and erythropoiesis. Our aim was to better understand how hypoxia regulates iron acquisition for erythropoiesis in humans, a topic relevant to common hypoxia-related disorders. Forty-seven healthy volunteers participated in the HIGHCARE project. Blood samples were collected at sea level and after acute and chronic exposure to high altitude (3400-5400 m above sea level). We investigated the modifications in hematocrit, serum iron indices, erythropoietin, markers of erythropoietic activity, interleukin-6, and serum hepcidin. Hepcidin decreased within 40 hours after acute hypoxia exposure (P < .05) at 3400 m, reaching the lowest level at 5400 m (80% reduction). Erythropoietin significantly increased (P < .001) within 16 hours after hypoxia exposure followed by a marked erythropoietic response supported by the increased iron supply. Growth differentiation factor-15 progressively increased during the study period. Serum ferritin showed a very rapid decrease, suggesting the existence of hypoxia-dependent mechanism(s) regulating storage iron mobilization. The strong correlation between serum ferritin and hepcidin at each point during the study indicates that iron itself or the kinetics of iron use in response to hypoxia may signal hepcidin down-regulation. The combined and significant changes in other variables probably contribute to the suppression of hepcidin in this setting.

Hepcidin in beta-thalassemia

Iron overload is the principal cause of morbidity and mortality in beta-thalassemia with or without transfusion dependence. Iron homeostasis is regulated by the hepatic peptide hormone hepcidin. Hepcidin controls dietary iron absorption, plasma iron concentrations, and tissue iron distribution. A deficiency in this hormone is the main or contributing factor of iron overload in iron-loading anemias such as beta-thalassemia. Hepcidin deficiency results from a strong suppressive effect of the high erythropoietic activity on hepcidin expression. Although in thalassemia major patients iron absorption contributes less to the total iron load than transfusions, in non-transfused thalassemia, low hepcidin, and the consequent hyperabsorption of dietary iron is the major cause of systemic iron overload. Hepcidin diagnostics and future therapeutic agonists may help in management of patients with beta-thalassemia.

A dual-monoclonal sandwich ELISA specific for hepcidin-25

BACKGROUND

Hepcidin, a key regulator of iron metabolism, binds to the iron transporter ferroportin to cause its degradation. In humans, hepcidin deficiency has been linked to hemochromatosis and iron overload, whereas increased concentrations have been reported in anemia of cancer and chronic disease. There is currently an unmet clinical need for a specific immunoassay with a low limit of quantification to measure serum concentrations of hepcidin-25, the active form of the protein.

METHODS

We generated 2 antihepcidin-25 monoclonal antibodies and used them to build a sandwich ELISA. We correlated ELISA results to hepcidin-25 measurements by LC-MS and used ELISA to measure serum hepcidin-25 concentrations in normal individuals, cancer patients, and patients with rheumatoid arthritis.

RESULTS

The sandwich ELISA was highly specific for hepcidin-25, having a limit of quantification of 0.01 μg/L (10 pg/mL). Serum concentrations of hepcidin-25 measured by ELISA correlated with hepcidin-25 concentrations measured by using an independent LC-MS assay (r = 0.98, P < 0.001). Hepcidin-25 concentrations were increased in patients with cancer (median 54.8 μg/L, 25%-75% range 23.2-93.5 μg/L, n = 34) and rheumatoid arthritis (median 10.6 μg/L, 25%-75% range 5.9-18.4 μg/L, n = 76) compared with healthy individuals (median 1.20 μg/L, 25%-75% range 0.42-3.07 μg/L, n = 100).

CONCLUSIONS

The use of 2 monoclonal antibodies in a sandwich ELISA format provides a robust and convenient method for measuring concentrations of the active form of hepcidin. This ELISA should help to improve our understanding of the role of hepcidin in regulating iron metabolism.

Detection, evaluation, and management of iron-restricted erythropoiesis

Progress in our understanding of iron-restricted erythropoiesis has been made possible by important advances in defining the molecular mechanisms of iron homeostasis. The detection and diagnostic classification of iron-restricted erythropoiesis can be a challenging process for the clinician. Newer assays for markers of inflammation may allow more targeted management of the anemia in these conditions. The availability of new intravenous iron preparations provides new options for the treatment of iron-restricted erythropoiesis. This review summarizes recent advances regarding the detection, evaluation, and management of iron-restricted erythropoiesis.

Erythropoiesis and iron sulfur cluster biogenesis

Erythropoiesis in animals is a synchronized process of erythroid cell differentiation that depends on successful acquisition of iron. Heme synthesis depends on iron through its dependence on iron sulfur (Fe-S) cluster biogenesis. Here, we review the relationship between Fe-S biogenesis and heme synthesis in erythropoiesis, with emphasis on the proteins, GLRX5, ABCB7, ISCA, and C1orf69. These Fe-S biosynthesis proteins are highly expressed in erythroid tissues, and deficiency of each of these proteins has been shown to cause anemia in zebrafish model. GLRX5 is involved in the production and ABCB7 in the export of an unknown factor that may function as a gauge of mitochondrial iron status, which may indirectly modulate activity of iron regulatory proteins (IRPs). ALAS2, the enzyme catalyzing the first step in heme synthesis, is translationally controlled by IRPs. GLRX5 may also provide Fe-S cofactor for ferrochelatase, the last enzyme in heme synthesis. ISCA and C1orf69 are thought to assemble Fe-S clusters for mitochondrial aconitase and for lipoate synthase, the enzyme producing lipoate for pyruvate dehydrogenase complex (PDC). PDC and aconitase are involved in the production of succinyl-CoA, a substrate for heme biosynthesis. Thus, many steps of heme synthesis depend on Fe-S cluster assembly.

Growth differentiation factor 15 in erythroid health and disease

PURPOSE OF REVIEW

Growth differentiation factor 15 (GDF15) was identified as a hepcidin-suppression factor that is expressed at high levels in patients with ineffective erythropoiesis. This review addresses the regulation, expression and potential functions of GDF15 in the context of erythroid biology.

RECENT FINDINGS

GDF15 expression during late erythroid differentiation was discovered as part of an erythroblast transcriptome project. As GDF15 expression is associated with cellular stress or apoptosis, further investigation of the cytokine was focused upon its involvement in ineffective erythropoiesis. Remarkably high serum levels were detected in patients with thalassemia syndromes, congenital dyserythropoiesis and some acquired sideroblastic anemias. High-level GDF15 expression is not a feature of normal erythropoiesis, or erythroid recovery after bone-marrow transplantation. As GDF15 is a transforming growth factor-beta superfamily member, it was investigated as an effector of ineffective erythropoiesis that suppresses hepcidin expression despite iron overloading.

SUMMARY

In contrast to the low levels of GDF15 expressed during normal erythropoiesis, ineffective erythropoiesis causes high-level expression of GDF15. In patients with thalassemia and related anemias, GDF15 expression may contribute to iron overloading or other features of the disease phenotype.

Down-regulation of hepcidin resulting from long-term treatment with an anti-IL-6 receptor antibody (tocilizumab) improves anemia of inflammation in multicentric Castleman disease

Dysregulated production of hepcidin is implicated in anemia of inflammation, whereas interleukin-6 (IL-6) is a major inducer of hepcidin production. Overproduction of IL-6 is responsible for pathogenesis of multicentric Castleman disease (MCD), a rare lymphoproliferative disorder accompanied by systemic inflammatory responses and anemia. In this study, we investigated the roles of hepcidin and IL-6 in anemia of inflammation and the long-term effects of anti-IL-6 receptor antibody (tocilizumab) treatment on serum hepcidin and iron-related parameters in MCD patients. We found that tocilizumab treatment resulted in a rapid reduction of serum hepcidin-25 in 5 of 6 MCD patients. Long-term reductions, accompanied by progressive normalization of iron-related parameters and symptom improvement, were observed in 9 of 9 cases 1.5, 3, 6, and 12 months after the start of tocilizumab treatment. In in vitro experiments, IL-6-induced up-regulation of hepcidin mRNA in hepatoma cell lines was completely inhibited by tocilizumab but increased in the presence of patients' sera. Our results suggest that, although multiple factors affect serum hepcidin levels, IL-6 plays an essential role in the induction of hepcidin in MCD. This accounts for the long-term ameliorative effect of IL-6 blockage with tocilizumab on anemia by inhibiting hepcidin production in MCD patients.

Quantitation of hepcidin in serum using ultra-high-pressure liquid chromatography and a linear ion trap mass spectrometer

Hepcidin is a peptide hormone that functions as a key regulator of mammalian iron metabolism. Biological levels are increased in end-stage renal disease and during inflammation but suppressed in hemochromatosis. Thus hepcidin levels have diagnostic importance. This study describes the development of an analytical method for the quantitative determination of the concentration of hepcidin in clinical samples. The fragmentation of hepcidin was investigated using triple quadrupole and linear ion trap mass spectrometers. A standard quantity of a stable isotopically labelled hepcidin internal standard was added to serum samples. Extraction was performed by protein precipitation and weak cation-exchange magnetic nanoparticles. Chromatography was carried out on sub 2 microm particle stationary phase, using ultra-high-pressure liquid chromatography and a linear ion trap for quantitation. The lower limit of quantitation was 0.4 nmol/L with less than 20% accuracy and precision. The mean hepcidin concentration in sera for controls was 4.6 +/- 2.7 nmol/L, in patients with sickle cell disease, 7.0 +/- 8.9 nmol/L; in patients with end-stage renal disease, 30.5 +/- 15.7 nmol/L; and patients with penetrant hereditary hemochromatosis, 1.4 +/- 0.8 nmol/L.

Iron Loading and Overloading due to Ineffective Erythropoiesis

Erythropoiesis describes the hematopoietic process of cell proliferation and differentiation that results in the production of mature circulating erythrocytes. Adult humans produce 200 billion erythrocytes daily, and approximately 1 billion iron molecules are incorporated into the hemoglobin contained within each erythrocyte. Thus, iron usage for the hemoglobin production is a primary regulator of plasma iron supply and demand. In many anemias, additional sources of iron from diet and tissue stores are needed to meet the erythroid demand. Among a subset of anemias that arise from ineffective erythropoiesis, iron absorption and accumulation in the tissues increases to levels that are in excess of erythropoiesis demand even in the absence of transfusion. The mechanisms responsible for iron overloading due to ineffective erythropoiesis are not fully understood. Based upon data that is currently available, it is proposed in this review that loading and overloading of iron can be regulated by distinct or combined mechanisms associated with erythropoiesis. The concept of erythroid regulation of iron is broadened to include both physiological and pathological hepcidin suppression in cases of ineffective erythropoiesis.

Targeting the hepcidin-ferroportin axis in the diagnosis and treatment of anemias

The hepatic peptide hormone hepcidin regulates dietary iron absorption, plasma iron concentrations, and tissue iron distribution. Hepcidin acts by causing the degradation of its receptor, the cellular iron exporter ferroportin. The loss of ferroportin decreases iron flow into plasma from absorptive enterocytes, from macrophages that recycle the iron of senescent erythrocytes, and from hepatocytes that store iron, thereby lowering plasma iron concentrations. Malfunctions of the hepcidin-ferroportin axis contribute to the pathogenesis of different anemias. Deficient production of hepcidin causes systemic iron overload in iron-loading anemias such as beta-thalassemia; whereas hepcidin excess contributes to the development of anemia in inflammatory disorders and chronic kidney disease, and may cause erythropoietin resistance. The diagnosis of different forms of anemia will be facilitated by improved hepcidin assays, and the treatment will be enhanced by the development of hepcidin agonists and antagonists.

Evidence for a lack of a direct transcriptional suppression of the iron regulatory peptide hepcidin by hypoxia-inducible factors

Background

Hepcidin is a major regulator of iron metabolism and plays a key role in anemia of chronic disease, reducing intestinal iron uptake and release from body iron stores. Hypoxia and chemical stabilizers of the hypoxia-inducible transcription factor (HIF) have been shown to suppress hepcidin expression. We therefore investigated the role of HIF in hepcidin regulation.

Methodology/Principal Findings

Hepcidin mRNA was down-regulated in hepatoma cells by chemical HIF stabilizers and iron chelators, respectively. In contrast, the response to hypoxia was variable. The decrease in hepcidin mRNA was not reversed by HIF-1α or HIF-2α knock-down or by depletion of the HIF and iron regulatory protein (IRP) target transferrin receptor 1 (TfR1). However, the response of hepcidin to hypoxia and chemical HIF inducers paralleled the regulation of transferrin receptor 2 (TfR2), one of the genes critical to hepcidin expression. Hepcidin expression was also markedly and rapidly decreased by serum deprivation, independent of transferrin-bound iron, and by the phosphatidylinositol 3 (PI3) kinase inhibitor LY294002, indicating that growth factors are required for hepcidin expression in vitro. Hepcidin promoter constructs mirrored the response of mRNA levels to interleukin-6 and bone morphogenetic proteins, but not consistently to hypoxia or HIF stabilizers, and deletion of the putative HIF binding motifs did not alter the response to different hypoxic stimuli. In mice exposed to carbon monoxide, hypoxia or the chemical HIF inducer N-oxalylglycine, liver hepcidin 1 mRNA was elevated rather than decreased.

Conclusions/Significance

Taken together, these data indicate that hepcidin is neither a direct target of HIF, nor indirectly regulated by HIF through induction of TfR1 expression. Hepcidin mRNA expression in vitro is highly sensitive to the presence of serum factors and PI3 kinase inhibition and parallels TfR2 expression.